The present invention is generally directed to the manufacture of printed circuit boards and, in particular, to a fixture for facilitating the manufacture of printed circuit boards bearing surface-mounted (i.e., SMT) components.
One common technique used in the manufacture of printed circuits involves the application of components to one side, or surface, of the printed circuit board, followed by connection of the leads associated with the applied components to a corresponding series of pads formed on the surface of the printed circuit board. To ensure a proper. result, care must be taken to correctly locate such SMT components on the printed circuit board so that the leads of each component are correctly aligned with their corresponding pads. Steps are then taken to appropriately connect the leads to the corresponding pads, using soldering techniques which are themselves known.
The improvements of the present invention are primarily directed to a particular step of the manufacturing process which is used to assemble a printed circuit board bearing SMT components, the step of soldering the plural leads of the components to the pads on the printed circuit board. In practice, one of the difficulties encountered in applying SMT components to a printed circuit board has been to ensure that all of the leads associated with a particular component are approximately co-planar with the corresponding pads on the printed circuit board. This co-planarity is important in preparing the leads for soldered connection to the corresponding pads. Moreover, it has been found that the difficulties encountered in aligning the leads to their corresponding pads are magnified by situations in which components (1) extend over the edge of the printed circuit board, (2) are placed on solid solder rather than on solder paste, or (3) have locating projections.
A variety of fixtures have been used in the industry to manufacture (solder) printed circuit (i.e., electronic card) assemblies. When used in conjunction with a wave solder process, such fixtures often include weights or clamps to press down on the body of the SMT component while the soldering takes place. Such weights and clamps are not typically used on the fixtures, however, when reflow soldering is employed. In addition, the fixtures have not been found to be entirely satisfactory for ensuring proper alignment of the plural leads of an SMT component with each of the corresponding pads which are to receive the soldered leads. This problem is exacerbated as the number of leads associated with a given SMT component increases, responsive to advances in technology, requiring accommodation of a greater number of leads (and a correspond number of pads) in a limited amount of space.
Therefore, a primary object of the present invention is to provide an improved fixture for use in soldering the leads of SMT components to the corresponding pads of a printed circuit board. Another object of the present invention is to provide an alignment fixture for ensuring that all of the leads of an SMT component are approximately co-planar with the corresponding pads of the printed circuit board after their soldered connection.
These and other objects which will become apparent are achieved in accordance with the present invention by providing a fixture for aligning the leads of SMT components with the corresponding pads of the printed circuit board which is to receive such components. The fixture includes appropriate clamps for applying forces directly to the leads of the SMT components, alone or in conjunction with the clamping forces which are traditionally applied to the body of the component. In this way, forces are applied to the leads sufficient to seat each of the leads on its corresponding pad, thereby ensuring that the leads are approximately co-planar before soldering.
Preferably, the advantages of the present invention are accomplished by providing the fixture with an appropriate series of rocker-type clamps, each having an edge for engaging the several leads associated with a particular SMT component to be joined to the printed circuit board. The clamp is preferably constructed of a non-wettable material, such as titanium, so that the edge of the clamp will not be wet by the molten solder during the soldering procedure. The edge is preferably thin so that the clamping surface in contact with a particular lead is minimized. Openings are advantageously provided in the clamp, above each of the leads of the SMT component, to allow radiant and convective heat transfer to each of the solder joints. Such a clamp further allows SMT components to be soldered in a reflow oven, which would otherwise have required either a manual process or special equipment.